The present invention relates to urethane di(meth)acrylate derivatives of 1,3-bis(1-isocyanato-1-methylethyl)benzene and to dental materials on the basis of these substances.
Urethane (meth)acrylates find practical applications inter alia as a constituent of adhesives, coatings and dental materials (R. Holman (Pub.), U.V. and EB. Curing Formulation for Printing Inks, Coatings and Paints, SITA-Technology, London 1984, 27; J. P. Foussier, J. F. Rabek (Pub.), Radiation Curing in Polymer Science and Technology, Vol. IV, Elsevier Applied Science, London and New York 1993, 387). A monomer which is used particularly frequently in the dental field is 7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5, 12-diazahexadecan-1,16-diyldimethacrylate (UDMA) which is accessible by reaction of one mole of 2,2,4-trimethylhexa-methylene diisocyanate with two moles of 2-hydroxyethyl methacrylate (HEMA) (cf. e.g. DE 195 44 671).
However, the refractive index of UDMA, at nD=1.483, is clearly different from the refractive index of customary dental filling materials (ca. 1.52 to 1.55), so that UDMA and other aliphatic urethane dimethacrylates are frequently combined with bis-GMA, the addition product of methacrylic acid and bisphenol-A-diglycidyl ether (refractive index nD=1.549) to match the refractive index to the filler (cf. e.g. DE OS 24 11 760). The mechanical properties of the materials can also be improved by the addition of bis-BMA.
Through the matching of the refractive indices, a greater through-curing depth of the dental materials upon photopolymerization is achieved, but, because of the hydroxyl groups present, bis-GMA encourages the water absorption of the materials, which leads to a reduced durability under moist conditions. Moreover, bis-GMA frequently contains impurities, which are hard to remove, of bisphenol-A which has a pronounced oestrogenic action.
In addition to UDMA, the use of other di(meth)acrylate urethanes has been described. M. G. Buonocore and C. A. Casciani, New York State Dental Journal 35 (1969) 135, describe for example addition products of two moles of HEMA and one mole each of 2,4-toluylene diisocyanate, hydrogenated diphenylmethane diisocyanate, naphthalene diisocyanate or hexanemethylene diisocyanate. These are all crystalline compounds which can be processed to dental materials only together with liquid monomers.
U.S. Pat. No. 4,400,159 discloses urethane diacrylates which are obtained by reaction of aliphatic and aromatic diisocyanates with 3-methacrylol-2-hydroxypropyl esters. However, the substances tend to become discoloured, and the aromatic derivatives are crystalline compounds. These monomers are preferably combined with bis-GMA.
DE 195 44 671 A1 discloses urethane (meth)acrylates with cyclic carbonate groups which are said to show an increased speed of polymerization and a lower sensitivity to polymerization inhibition by oxygen.
U.S. Pat. No. 4,952,241, EP 0 254 185 B1, U.S. Pat. No. 4,904,750 and EP 0 658 582 A1 disclose prepolymeric (meth)acryl urethane derivatives which can be used above all as flexibilizing monomers or dilution monomers in combination with bis-GMA.
U.S. Pat. No. 4,386,912 relates to dental filling materials on the basis of tetrafunctional urethane acrylate monomers, which can be prepared by reaction of glycerol dimethacrylate with aromatic or aliphatic diisocyanates. Aliphatic diisocyanates are preferred as regards the coloration of the cured product.
B. Nabeth, J. F. Gerard, J. P. Pascault, J. Appl. Polym. Sci. 60 (1996) 2113, describe the synthesis of polyurethane (meth)-acrylates on the basis of polycaprolactone macrodiols using 1,3-bis (1-isocyanato-1-methylethyl)benzene (TMXDI). Low-molecular-weight urethane di(methacrylate) derivatives on the basis of TMXDI are not known at present.
The object of the invention is to prepare urethane di(meth)-acrylate derivatives capable of flowing and capable of polymerization, whose refractive index is compatible with that of customary dental filling materials, which do not to tend towards discolorations and which can replace bis-GMA in dental materials without impairing the mechanical properties of the materials.
This object is achieved by urethane di(meth)acrylate derivatives of 1,3-bis(1-isocyanato-1-methylethyl)benzene according to Formula (I), 
in which
R is hydrogen or a straight-chained C1-C8 alkyl radical, preferably hydrogen, methyl, ethyl, propyl, butyl or hexyl, quite particularly preferably hydrogen or methyl and
X and Y independently of each other stand for 
xe2x80x83in which
R1 is a substituted or unsubstituted C6- to C12-aryl or C7- to C16-alkyl aryl or C7- to C12-aryl alkyl radical and
R2 is hydrogen, a C1- to C5-alkyl or a substituted or unsubstituted C6- to C12-aryl radical;
R3 is hydrogen or a methyl radical and
R4 is a C1- to C8-alkylene radical which can be interrupted by oxygen atoms, or is a phenylene radical;
R5 is hydrogen or a methyl radical,
R6 is a substituted or unsubstituted C6- to C2-aryl or C7- to C16-alkyl aryl or C7- to C12-aryl alkyl radical,
Z is xe2x80x94COxe2x80x94 or a chemical bond and
W stands for oxygen, sulphur or NR 7, whereby
R7 is hydrogen or a straight-chained C1- to C6-alkyl radical.
R and R7 preferably have the same meaning.
The aromatic groups both of the aryl and of the alkyl aryl radicals can be singly or repeatedly, preferably singly, substituted. Preferred substituents are halogen, in particular bromine, xe2x80x94OCH3, xe2x80x94OH, xe2x80x94CN, xe2x80x94CH3, xe2x80x94C2H5, xe2x80x94NO2xe2x80x94COOH and xe2x80x94COOCH3.
Preferred C7- to C12-aryl alkyl radicals are benzyl, a methyl-benzyl, xcex1,xcex1-dimethylbenzyl and xcex1,xcex1-diethylbenzyl, in particular benzyl.
Preferred definitions which can be chosen independently of one another are:
R1 hydrogen or xe2x80x94CH3,
R2 xe2x80x94CH3, xe2x80x94C2H5, a benzyl or phenyl radical,
R3 hydrogen or a methyl radical,
R4 an ethylene, propylene, triethylene, butylene or phenylene radical,
R5 a methyl radical,
R6 a benzyl, phenyl or substituted phenyl radical,
W oxygen, sulphur or NH,
Z xe2x80x94COxe2x80x94 or a chemical bond and/or
R7 hydrogen.
Quite particularly preferred definitions which can be chosen independently of each other are:
R1 hydrogen,
R2 hydrogen, a benzyl or phenyl radical,
R3 a methyl radical,
R4 an ethylene, triethylene or propylene radical,
R5 a methyl radical,
R6 a benzyl radical,
W oxygen,
Z xe2x80x94COxe2x80x94 and/or
R7 hydrogen.
Furthermore, urethane di(meth)acrylate derivatives in which X and Y have the same meaning are preferred.
Particularly preferred urethane di(meth)acrylate derivatives are: 
Furthermore, compounds according to Formula I in which R and R3 independently of each other, are hydrogen or methyl and R4 is ethylene or propylene are particularly preferred.
The urethane di(meth)acrylate derivatives according to the invention of formula (I) can be prepared by reaction of commercial 1,3-bis(1-isocyanato-1-methylethyl)benzene (TMXDI) with corresponding hydroxy(meth)acrylates Xxe2x80x94OH or Yxe2x80x94OH and optionally subsequent alkylation of the formed adducts for example with a dialkyl sulphate. 
The preparation of the hydroxy(meth)acrylates Xxe2x80x94OH and Yxe2x80x94OH can take place in a manner known per se (cf. e.g. C. Ferri, Reaktionen der organischen Synthese [Organic Synthesis Reactions], G. Thieme Verlag, Stuttgart 1978). The Baylis-Hillman reaction, catalysed by tertiary amines, of acrylates with aldehydes according to the reaction equation 
in which R1 and R2 have the meaning given above, is preferred. For example, 2-hydroxymethyl acrylic acid benzyl ester can be prepared by reaction of acrylic acid benzyl ester with formaldehyde:

Further preferred is the unstoichiometric esterification of dihydroxy compounds with (meth)acrylic acid or (meth)acrylic acid chloride according to the reaction equation 
in which R3 and R4 have the meaning given above and U=Cl or is OH. For example, 4-hydroxyphenyl methacrylate is accessible by reaction of hydroquinone with methacrylic acid chloride:

Moreover, the synthesis of suitable hydroxy(meth)acrylates can take place by reaction of glycidyl (meth)acrylate with O-nucleophilic reagents, such as alcohols, phenols or carboxylic acids according to the reaction equation 
in which R5 and R6 have the meaning given above. For example, 1-benzylcarbonyloxy-2-hydroxypropyl methacrylate can be obtained by reaction of phenylacetic acid with glycidyl methacrylate:

The urethane di(meth)acrylate derivatives according to the invention are suitable in particular for the production of polymers, adhesives and dental materials, such as filling composites, dental adhesives and fixing cements, with the urethane di(meth)acrylates acting as crosslinkers.
Derivatives with a refractive index of nD=1.50 to 1.60, in particular 1.50 to 1.55, are preferred for the production of dental materials.
For the polymerization, the compounds according to the invention are mixed with initiators for radical polymerization and optionally additional radically polymerizable monomers and fillers plus other auxiliaries.
Suitable initiators are described for example in the Encyclopedia of Polymer Science and Technology, Vol. 13, Wiley-Intersci. Pub., New York etc. 1988, p. 754 et seq. Preferred initiators for cold polymerization are azo compounds such as azobis(isobutyronitrile) (AIBN) or azobis(4-cyanovaleric acid) or peroxides, such as dibenzoyl peroxide, dilauroyl peroxide, tert.-butyl peroctoate, tert.-butyl perbenzoate or di-(tert.-butyl) peroxide.
Benzpinacol and 2,2xe2x80x2-Di(C1-C8-alkyl)benzpinacols in particular are suitable as initiators for hot curing.
Suitable photoinitiators for the UV or visible range are described by J. P. Foussier, J. F. Rabek (Pub.), Radiation Curing in Polymer Science and Technology, Vol. II, Elsevier Applied Science, London and New York 1993, pages 155 to 237. Preferred photoinitiators are benzoin ethers, dialkyl benzil ketals, dialkoxyacetophenones, acylphosphinic oxides, xcex1-diketones, such as 10-phenanthrenequinone, diacetyl, furil, anisil, 4,4xe2x80x2-dichlorobenzil and 4,4xe2x80x2-dialkoxybenzil and camphor quinone.
Dibenzoyl peroxide, camphor quinone or acylphosphinic oxides are particularly suitable for the production of dental materials.
Difunctional crosslinker monomers are preferred as additional radically polymerizable monomers, with crosslinking bi- or higher-functional acrylates and methacrylates, such as for example UDMA, di- or triethylene glycol di(meth)acrylate (TEGDMA), decanediol di(meth)acrylate, trimethylol propane tri(meth)-acrylate, pentaerythritol tetra(meth)acrylate, butanediol (di)-methacrylate, 1,10-decanediol di(meth)acrylate diol di(meth)acrylate above all being suitable for producing adhesives or dental materials. These monomers are accessible by esterification of (meth)acrylic acid with suitable diols.
Organic as well as inorganic particles and fibres are suitable as fillers. Preferred inorganic fillers for producing dental materials are amorphous, spherical materials on the basis of mixed oxides from SiO2, ZrO2 and/or TiO2 with an average particle size of 0.005 to 2.0 xcexcm, preferably of 0.1 to 1 xcexcm, as are disclosed for example in DE-PS 32 47 800, microfine fillers, such as pyrogenic silica or precipitated silica, as well as macro- or mini-fillers, such as quartz, glass ceramic or glass powder with an average particle size of 0.5 to 20 xcexcm, as well as X-ray-opaque fillers, such as ytterbium trifluoride. The term mini-fillers is taken to mean fillers with a particle size of 0.5 to 1.5 xcexcm, and the term macro-fillers to mean fillers with a particle size of 10 to 20 xcexcm.
Glass, polyamide or carbon fibres can also be used as fillers. Suitable reinforcing fibres are described for example in the xe2x80x9cTaschenbuch der Kunststoff-Additivexe2x80x9d, R. Gachter, H. Muller, Carl Hanser Verlag, Munich and Vienna 1990, pages 617 to 662.
The compositions according to the invention can also if needed contain other auxiliaries such as solvents, in particular water, ethyl acetate or ethanol, stabilizers, UV absorbers, dyestuffs, pigments and/or slip agents. The term stabilizers is taken to mean substances which prevent premature polymerization and thus above all increase the storage stability of monomer mixtures and composites without however impairing the properties of the cured materials. Preferred stabilizers are hydroquinone monomethyl-ether (MEHQ) and 2,6-di-tert.-butyl-4 methylphenol (BHT).
Dental materials preferably have the following composition:
1 to 99 wt.-%, preferably 10 to 80 wt.-% and particularly preferably 20 to 70 wt.-% of one or more urethane di(meth)acrylates,
0 to 80 wt.-%, preferably 0 to 60 wt.-% and particularly preferably 0 to 50 wt.-% of one or more other radically polymerizable monomers,
0 to 90 wt.-% fillers and
0.01 to 5 wt.-%, preferably 0.01 to 2 wt.-% of an initiator for radical polymerization.
The filler content is crucially determined by the intended use and is preferably 0 to 20 wt.-% in the case of adhesives, preferably 20 to 60 wt.-% in the case of cements and 50 to 85 wt.-% in the case of filling composites.
The proportion of the other auxiliaries usually lies in the range from 100 ppm to 1.0 wt.-% in each case, and in the case of dyestuffs and pigments, depending on colouring capacity, also in the range from 10 ppm up to 1.0 wt.-%.
The dental materials according to the invention preferably contain no bis-GMA, but have mechanical properties which correspond in every respect to those of materials containing bis GMA. Under moist conditions, the materials according to the invention display clearly better mechanical properties than the materials containing bis-GMA.
The urethane di(meth)acrylate derivatives according to the invention are moreover also suitable for the production of other medical or technical, radically curing adhesives, cements and composites, such as for example surgical bone cements, contact lenses, adhesives for optical parts, UV-curable lacquers, coatings and covering materials and also matrix resins for composite materials.
The invention is explained in more detail in the following with reference to embodiments.